The present invention relates to high bypass, turbofan engines and, more particularly, to a method and apparatus for supporting a turbofan engine such that a load path is provided which does not react through the engine core, thus preventing detrimental influences on engine performance.
In conventional turbofan and high bypass fan engines, a major performance penalty is attributed to clearance loss associated with inlet lift loads. Such loads occur from air flow entering the nacelle structure and reacting against the upper lip of the nacelle. This reaction tends to try to rotate the engine about the front mount. However, since the engine rear mount prevents such rotation, the reaction results in some degree of bending moment transmitted through the nacelle structure. Such nacelle bending can result in reduced clearance between the rotating elements of the core engine and the engine case.
For a better appreciation of this inlet lift loading phenomenon, reference is made to FIG. 1 which illustrates a conventional turbofan engine 10 comprised of front engine section 12, core engine section 14, and rear engine section 16. Front engine section 12 is comprised of nacelle 18 which extends from an air inlet region 20 to a rear nacelle region 22 located to the aft of fan outlet guide vanes (OGV's) 24. Fan OGV's 24 are located radially inward from nacelle 18 and connected thereto in a perpendicular manner. Fan blades 26 connected to rotor disk 28 are positioned forward of the fan OGV's 24 and are housed by fan casing 30. Fan casing 30 connects to and is located radially inward of nacelle 18. To the aft of fan OGV's 24 is located forward mount location 32 which comprises a position where an engine mount (not shown) attaches to the engine frame 149. The engine mount connects to the engine frame 149 on one side, then connects on its other side to a pylon (not shown) which is typically attached to an airplane wing (not shown). Located to the aft of forward mount location 32 and radially inward of engine cowling 34 are stators 36 which are located radially outward of high pressure rotors 38. Rotors 38 are connected to the core section of the engine 14 by means of shaft 40. The engine core section is comprised of high pressure compressor 42, combustion chamber 44, and high pressure turbines 40. The low pressure turbine 46 is located aft of core section 14 and drives fan rotor 28 through shaft 151. Rear mount location 48 is located aft of the low pressure turbine 46 and represents the location for support frame 150, similar to the support frame located at the forward mount location. The rear mount support frame is connected to a frame mount (not shown) which is similar to the frame mount located at the forward mount location. The rear frame mount connects to a rear location of the pylon (not shown). Thus, engine cowling 34 is connected to the pylon, the rear mount location 48, and the forward mount location 32.
Attention is now directed to FIG. 2 and rear mount location 48 and forward mount location 32. Lift force (lift load) V.sub.l acting along length L.sub.l, which extends from a location forward of fan rotor blades 26 to fan OGV's 24, produces a large moment M.sub.R which is equal to the cross product of force V.sub.l and length L.sub.l, i.e., M.sub.R =V.sub.l .times.L.sub.l. Force V.sub.l causes a reactive force V.sub.R at forward mount location 32. In addition to causing reactive force V.sub.R, the lift load which produces moment M.sub.R results in force V.sub.m being applied to rear mount location 48. L.sub.E is the length from front mount location 32 to rear mount location 48. Therefore, V.sub.m .times.L.sub.E =M.sub.R =V.sub.l .times.L.sub.l. The respective forces acting upon forward and rear mount locations 32 and 48 produce bending in the engine core, i.e., in high pressure (HP) compressor 42, HP turbines 47, and LP turbine 46. Core engine deflection line 50 of FIG. 3 illustrates this bending.
In the prior art turbine engine exemplified in FIGS. 1-3, the HP rotors do not bend between their supporting bearings (not shown); therefore, there is a net clearance closure (at engine top vertical) and a net clearance opening (at engine bottom vertical) between rotor and stator. The stator is ground large in the closure area so rubs do not occur; however, what does occur is performance loss during flight conditions as a result of excessive clearance. This situation is not alleviated by increasing the fan bypass ratio. As fan bypass ratio increases, the core engines are of course reduced in size relative to a fixed fan size and sensitivities to clearance closure are high. Also, since the core is made smaller, the stiffness of the core is less and deflections will be even greater. Further, spatial changes between the stator and rotor are caused by the inlet load and relative stiffness of the outer case, fan frame, and bearing supports.